7.2 – Printed Circuit Boards and Board Level Troubleshooting

What’s a PCB?

Printed circuit board is the most common name but may also be called “printed wiring boards” or “printed wiring cards”. PCB is an acronym for printed circuit board. It is a board that has lines and pads that connect various points together. In the picture above, there are traces that electrically connect the various connectors and components to each other. A PCB allows signals and power to be routed between physical devices. Solder is the metal that makes the electrical connections between the surface of the PCB and the electronic components. Being metal, solder also serves as a strong mechanical adhesive.

Terminology

Now that you’ve got an idea of what a PCB structure is, let’s define some common terms that you may hear when dealing with PCBs:

• Annular ring – the ring of copper around a plated through hole in a PCB
• DRC – design rule check. A software check of your design to make sure the design does not contain errors such as traces that incorrectly touch, traces too skinny, or drill holes that are too small.
• Drill hit – places on a design where a hole should be drilled, or where they actually were drilled on the board. Inaccurate drill hits caused by dull bits are a common manufacturing issue.
• Finger – exposed metal pads along the edge of a board, used to create a connection between two circuit boards. Common examples are along the edges of computer expansion or memory boards and older cartridge-based video games.
• Pad – a portion of exposed metal on the surface of a board to which a component is soldered.
• Panel – a larger circuit board composed of many smaller boards which will be broken apart before use. Automated circuit board handling equipment frequently has trouble with smaller boards, and by aggregating several boards together at once, the process can be sped up significantly.
• Plane – a continuous block of copper on a circuit board, define by borders rather than by a path. Also commonly called a “pour”.
• Plated through hole – a hole on a board which has an annular ring and which is plated all the way through the board. May be a connection point for a through hole component, a via to pass a signal through, or a mounting hole.
• Pogo pin – spring-loaded contact used to make a temporary connection for test or programming purposes.
• Silkscreen – the letters, number, symbols, and imagery on a circuit board. Usually only one color is available, and resolution is usually fairly low.
• Slot – any hole in a board which is not round. Slots may or may not be plated. Slots sometimes add to add cost to the board because they require extra cut-out time.
• Soldermask – a layer of protective material laid over the metal to prevent short circuits, corrosion, and other problems. Frequently green, although other colors (SparkFun red, Arduino blue, or Apple black) are possible. Occasionally referred to as “resist”.
• Surface mount – construction method which allows components to be simply set on a board, not requiring that leads pass through holes in the board. This is the dominant method of assembly in use today, and allows boards to be populated quickly and easily.
• Thermal – a small trace used to connect a pad to a plane. If a pad is not thermally relieved, it becomes difficult to get the pad to a high enough temperature to create a good solder joint. An improperly thermally relieved pad will feel “sticky” when you attempt to solder to it, and will take an abnormally long time to reflow.
• Trace – a continuous path of copper on a circuit board.
• Via – a hole in a board used to pass a signal from one layer to another. Tented vias are covered by soldermask to protect them from being soldered to. Vias where connectors and components are to be attached are often untented (uncovered) so that they can be easily soldered.

Watch

Video: The basic principles of PCBs:

Troubleshooting PCBs

Circuits boards are becoming increasingly more complex resulting in more manufacturing errors or component failures. Therefore, you are likely going to come across a circuit board that needs repairing or replacing. Smaller boards can be repaired using a DMM, some simple tools and a methodological approach. More complex boards may require automated systems to help identify issues and make the required repairs. Truthfully, when a board stops working, you are more likely going to replace the whole board rather than take the time to identify and fix a broken board. Regardless, if you are repairing or replacing a board, it is still useful to understand why the board failed. Below, you can find some troubleshooting guidelines

Common Reasons Why PCBs fail

Tips for troubleshooting PCBs

Recreate: https://www.mclpcb.com/pcb-guide/

Recreate flowchart from: https://www.protoexpress.com/blog/how-to-troubleshoot-and-repair-your-pcb/ or https://www.nutsvolts.com/magazine/article/repairing-circuit-boards